The use of biofuels is becoming more and more popular around the world especially based upon concerns from limited petroleum resources, increasing energy demand, greenhouse gas emissions and related climate change concerns. In addition to producing petroleum derived fuels, the fuels can also be manufactured using carbon and hydrogen derived from organic biomass, such as vegetable oils, organic fats, and organic greases.
For example, biological oils and fats can be converted into diesel, naphtha and jet fuels using many different processes, such as hydro-deoxygenation and hydro-isomerization processes. Diesel fuel refers to a mixture of carbon chains that generally contain between 8 and 21 carbon atoms per molecule. Typically, diesel has a boiling point in the range of 180 to 380° C. (356 to 716° F.). The production of diesel fuel can be either petroleum-derived or biologically-sourced. Petroleum-derived diesel is produced from the fractional distillation of crude oil, refining products, or by conversion processes. On the other hand, biologically-sourced diesel fuel is derived from renewable feedstock, such as vegetable oils or animal fats.
The biologically-sourced diesel fuel is desirable for a variety of reasons. In addition to the ecological benefits of using biologically-sourced diesel fuel, there exists a market demand for such fuel. For diesel purchasers, the use of biologically-sourced diesel fuel can be promoted in public relations. Also, certain governmental policies may require or reward use of biologically-sourced fuels. Finally, fluctuation of crude oil prices is also a reason refiners may choose to produce biologically-sourced fuels. The biologically-sourced diesel fuel is usually classified into two categories, biodiesel and green diesel.
Biodiesel (also referred to as fatty acid methyl ester, or FAME) mainly consists of long-chain alkyl esters typically mono-alkyl ester products derived from a lipid feedstock. The chemical structure of biodiesel is distinctly different from petroleum-derived diesel, and therefore biodiesel has somewhat different physical and chemical properties from petroleum-derived diesel. For example, biodiesel has a much higher oxygen content than petroleum-derived diesel.
Green diesel (also referred to as renewable hydrocarbon diesel, hydroprocessed vegetable oils or HVO), on the other hand, is substantially the same chemically as petroleum-derived diesel, but green diesel is made from recently living biomass. Unlike biodiesel, which is an ester and has different chemical properties from petroleum diesel, green diesel is composed of long-chain hydrocarbons, and can be mixed with petroleum diesel in any proportion for use as transportation fuel. Green diesel resembles petroleum-derived diesel fuel and usually has a very low heteroatom (nitrogen, oxygen, sulfur) content. Green diesel can thus be produced to be indistinguishable from petroleum diesel. This is beneficial because no changes to fuel infrastructure or vehicle technology are required for green diesel and it may be blended in any proportion with petroleum-derived diesel fuel as it is stable, not oxygenated. Further, unlike FAME biodiesel technology which produces glycerin as a by-product, the production of green diesel generates valuable co-products like naphtha, liquefied petroleum gas components like propane and butane, and fuel gases like methane and ethane.
The production of green diesel from some biomasses, such as vegetable oils, consumes large amounts of hydrogen. In some areas, hydrogen is not abundantly available and therefore, reactions that require large amounts of hydrogen may be economically unviable. However, even if areas in which hydrogen is available, the required hydrogen is an added cost for a refiner. In addition to having high hydrogen demands, the decarboxylation, decarbonylation, and hydrodeoxygenation reactions associated with converting the triglycerides found in the oils into paraffins typically produce large amounts of water.
Three main reactions are involved in producing the n-paraffin in presence of hydrogen for jet/diesel production are:(Decarboxylation)CnH2n+1COOR+H2=CnH2n+2+CO2+RH;(Decarbonylation)CnH2n+1COOR+2H2=CnH2n+2+CO+H2O+RH; and,(Hydrodeoxygenation)CnH2n+1COOR+4H2═Cn+1H2(n+2)+2H2O+RH.
Thus, deoxygenation could be achieved either by removal as water with hydrogen (hydrodeoxygenation), as carbon dioxide (CO2) (decarboxylation), as carbon monoxide (CO) (decarbonylation). As will be appreciated, decarboxylation and decarbonylation will result in the loss of carbon from the produced paraffin as the ester group is removed as carbon dioxide and carbon monoxide, respectively. On the other hand, hydrodeoxygenation maintains the carbon on the produced paraffin—increasing liquid yield and water production. It is believed that hydrodeoxygenation is preferred over decarboxylation and decarbonylation based upon the cost of the additional hydrogen consumption being less than the increased liquid yield (based upon current cost assumptions). Over and above these three key reactions, any double bonds present in the fatty acid side chains also undergo hydrogenation to produce saturated fatty acid side chains.
Therefore, it would be desirable to have one or more processes that allow for effective and efficient conversion of triglycerides into paraffins which reduces the amount of decarboxylation and decarbonylation.